1. Field of the Invention
The invention relates to a ceramic green sheet and a ceramic substrate.
2. Description of the Related Art
Recently, in mobile communication appliances and portable communication terminals such as a cell phone, a circuit module using a multilayer ceramic substrate has been employed in place of a conventional module which mounts capacitors and inductors on the surface of a printed board. With respect to the multilayered ceramic substrate, a wiring line, an L component, and a C component are formed by forming prescribed conductive patterns to actualize high density wiring. Use of the multilayer ceramic substrate makes it possible to decrease the number of inductors and capacitors to be mounted as discrete components (surface mount devices), so that the circuit module can be miniaturized.
Materials to be used for the multilayered ceramic substrate can be classified into mainly two types; HTCC (High Temperature Co-fired Ceramics) and LTCC (Low Temperature Co-fired Ceramics). Al2O3, AlN, SiC or the like may be used for materials usable as HTCC and these materials are produced by firing at a temperature as high as 1500° C. or higher. Therefore, Ag or Cu, which has a low melting point and low resistance although having high material strength, cannot be used for the conductive material to be used for an inner layer of the substrate and Mo or W, which has a high melting point and high resistance, has to be used. Accordingly, the conductor loss becomes high and although usable for a vehicular circuit substrate and low frequency sensor substrate, HTCC are difficult to be used for a high frequency module for a cellular phone or the like.
On the other hand, LTCC are possible to be sintered at a low temperature about 950° C. to 1000° C. so as not to melt Ag or Cu by combining a large quantity, about 50%, of a low melting point glass material with mainly ceramic raw materials (filler). Since simultaneous firing with a high conductivity noble metal material such as Ag and Cu is possible, the conductor loss can be lowered and LTCC can be usable for a high frequency module for a cell phone or the like. Further, due to decrease of the firing temperature, the energy to be consumed at the time of production can be reduced.
However, since LTCC contain a large quantity of glass, they are disadvantageous in the mechanical strength and chemical resistance as compared with HTCC. Particularly, the multilayer ceramic substrate tends to be made thinner as mobile appliances have been made more miniaturized and multi-functional and therefore insufficient strength of the substrate possibly causes a serious problem in the reliability of a product. To solve such a problem, addition of TiO2 and CuO or Nb2O5 and CuO has been proposed as a promoter for promoting firing in place of glass for materials having a lowered firing temperature without so much decreasing the content of Al2O3 (Japanese Patent No. 3220360, Japanese Patent Application Laid-Open (JP-A) No. 2004-256384, 9-67159, 9-235154, 9-278516, 2003-95732, and 2005-145722).
However, in the case of using the above-mentioned additive, the sinter-ability at a temperature as low as 1000° C. or lower at which Ag or Cu is usable as a wiring material is still insufficient. Accordingly, a costly Ag—Pd material or the like has to be used as a wiring material and LTCC are not so easily made usable for a high frequency module of mobile appliances and a substrate in which passive elements are mounted with high density.